DEVELOPERS: drop Stephan Hoffmann

His e-mail is no longer working:

<stephan.hoffmann@ext.grandcentrix.net>: host aspmx.l.google.com[74.125.133.26]
    said: 550-5.2.1 The email account that you tried to reach is disabled.
    Learn more at 550 5.2.1  https://support.google.com/mail/?p=DisabledUser


    o3si10331209wre.302 - gsmtp (in reply to RCPT TO command)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>